Analogue synthesiser circuits, being closely related to analogue
computer building blocks, often rely on the interaction of several
semiconductor devices to realise a particular function. In such
precision circuits the temperature dependence of semiconductor
parameters has to be taken into consideration, as device
temperature is likely to vary during operation. A means by which
functionally related semiconductor devices and compensation
components can be held at uniform temperature will usually have
to be provided.

In integrated circuits thermal coupling is inherent in the design
as the semiconductor junctions are implanted onto a common silicon
chip. Back in the days before integrated circuits were commonly
available, critical discrete circuit blocks would often be
encapsulated in potting compound to ensure temperature independent
performance. Apart from the technical objectives, encapsulation
provided a means by which proprietary ideas could be kept secret.

Circuit Modularization

Early ARP products such as the modules of the model 2500 synthesiser
had some of the basic circuit blocks encapsulated. Typical examples
of such circuits are the exponential converters of the oscillators,
and the variable gain cells of the Multimode Filter. Also, early
keyboard units made use of an encapsulated discrete FET-input op-amp
module as key control voltage buffer.

For the model 2600, however, ARP changed to a more compact and
cost-effective modularization scheme - complete synth module cores
would be built into little encapsulated plastic packages. As a
result, you only needed a minimum amount of support circuitry,
mainly panel controls and trimmers, to make completely functional
synthesiser modules such as oscillators, filters, and envelope
generators. Incidentally, ARP (or Tonus, as they were called back
then) made an effort to sell these circuit modules separately to
experimenters and educational facilities,
"Encapsulated Function Generators For
Use In Electronic Music Systems" [Note 1], as they were marketed
in the early 1970s.

Early encapsulated circuit blocks typically consisted of a small
circuit board enclosed in a plastic case filled with epoxy potting
compound. The simple circuit blocks used in the model 2500 measured
1.125" square, while the 2600 submodules were 2" square. The obvious
disadvantage to having circuits potted in epoxy is that they cannot
easily be repaired, as the components are inaccessible through the
hard potting material. There have been reports of people using special
organic solvents to clear the potting material, but this is a very
slow and potentially hazardous process - not to be recommended.

Sometime in 1972 or 1973, ARP changed from monolithic potting to a
two-layer encapsulation scheme. Module cases were now filled with
silicone rubber so that the circuit board and components were covered,
then sealed with a layer of epoxy, see figure 1. These modules are
repairable, although clearing off the silicone rubber around the
components is tedious and time-consuming. See the section on
opening encapsulated modules.

Figure 1.Cross-section of two-layer encapsulated module.

During the mid-seventies, ARP stopped encapsulating most of their
modules. Putting special metal clips around transistor pairs fulfilled
thermal coupling requirements for low-drift operation. Also, the
increased use of integrated circuit transistor arrays made
encapsulation schemes redundant. However, some of the voltage
controlled filter modules did remain encapsulated up until around
1976 by which time they had been replaced with the newer 4072/4075
design. The reasons for maintaining encapsulation were probably more
strategic than technical, as the core of some of the filter circuits
infringed on a Moog patent.

While the 2600 and Soloist (2700) were extensively designed around
submodules, later models omitted this scheme in order to keep costs
down. The Odyssey (2800) had three large circuit boards that
accommodated the main electronics and front panel controls -- only
the voltage controlled filter circuit took the form of a submodule.
Subsequent models would follow this scheme, and the smallest synths
in the range, the Axxe (2300) and Solus, were practically
single-board, submodule-less designs.